Can humans smell sex pheromones?

Dr Alfredo Sansone MRSB, research associate at University College London, introduces us to sex pheromones and scientists’ 40 year quest to discover if humans can smell them.

You might have heard about pheromones in the news or in some advertisements claiming that a perfume will make you irresistible, however, many people don’t know what pheromone means. The definition of pheromone dates back to 1930s, when they were first discovered in insects. At the time it was found that a molecule, or a mix of molecules, released by an individual can affect the behaviour of another individual of the same species, often triggering a sexual response. These molecules normally function via the sense of smell. In insects the effect of a pheromone can be very straightforward, and even different for the two sexes, meaning that the same molecule can inhibit mating behaviour in a male fly, but promote mating in a female.

As one might imagine, the situation is more complicated in mammals, due to their higher brain complexity. For instance, scented filter paper was often used for insect’s copulatory behaviour studies, however it is difficult to imagine a mouse attempting copulation with a piece of paper. Although mammals’ behavioural responses are not as simple, the effect of a pheromone can be as striking as the ‘Bruce effect’, where a pregnant mouse can prematurely terminate the pregnancy if exposed to unfamiliar male pheromones.

When it comes to humans, dealing with pheromones and behaviours is even more challenging, because of the complex nature of our social and sexual behaviours. In a famous study, researchers asked subjects to smell t-shirts that were previously worn by males and females for several days, and found that 70-80% of the subjects could identify the person’s sex based on the odour from the t-shirt. However, later it was found that people tend to associate stronger unpleasant odours with males, unveiling an unconscious bias in the ‘smelly t-shirt’ experiment. This shows how experiences can influence our choices or behaviours, making scientific testing and data interpretation extremely complicated.

Recently, a nipple secretion from lactating mothers, which induces suckling in neonates, has been proposed as a new human pheromone. In my opinion this is a good example of pheromone-induced behaviour, because there is no learning or experience bias involved. In other words, newborns have no idea how to get food from a nipple, but smelling this secretion will trigger a specific innate behaviour.

The big question is; can humans smell sex pheromones? The answer is: we do not know yet. There are some candidate molecules, but none of them are widely accepted by the scientific community. Many studies proposing new putative pheromones have been largely criticised, mainly due to faults in the experimental approach. Some scientists are even reconsidering the very definition of pheromone, as the original one was based on how some chemicals trigger unequivocal behaviours in insects, and this might not be applicable to humans. Perhaps pheromones can affect our physiological state or mood in more subtle ways that are not easy to identify in a simple scientific test.